Minor: reserve space for output views in ByteViewArrayDecoderDictionary

[alamb]

Which issue does this PR close?

• Broken out of Experimental parquet decoder with first-class selection pushdown support #6921 to make that PR smaller and easier to review

Rationale for this change

Reserving sufficient space before appending views potentially saves an allocation

What changes are included in this PR?

Reserve sufficient space for the views

Are there any user-facing changes?

Performance is if anything worse 🤔

++ critcmp main alamb_reserve_views
group                                                                          alamb_reserve_views                    main
-----                                                                          -------------------                    ----
arrow_array_reader/StringViewArray/dictionary encoded, mandatory, no NULLs     1.01    244.9±0.37µs        ? ?/sec    1.00   243.0±23.88µs        ? ?/sec
arrow_array_reader/StringViewArray/dictionary encoded, optional, half NULLs    1.21    265.1±4.33µs        ? ?/sec    1.00    219.4±1.12µs        ? ?/sec
arrow_array_reader/StringViewArray/dictionary encoded, optional, no NULLs      1.04    247.5±0.40µs        ? ?/sec    1.00    237.0±3.12µs        ? ?/sec
arrow_array_reader/StringViewArray/plain encoded, mandatory, no NULLs          1.00    388.2±1.69µs        ? ?/sec    1.04    402.1±1.19µs        ? ?/sec
arrow_array_reader/StringViewArray/plain encoded, optional, half NULLs         1.09    337.6±1.74µs        ? ?/sec    1.00    308.4±0.96µs        ? ?/sec
arrow_array_reader/StringViewArray/plain encoded, optional, no NULLs           1.00   397.0±21.83µs        ? ?/sec    1.03    408.9±1.35µs        ? ?/sec